Process and apparatus for separating gas mixtures



Oct. 23, 19 v. E. FIRST ETAL PROCESS AND APPARATUS FOR SEPARATING GASMIXTURES Original Filed Jan. 4, 1956 INVENTORS PHILIP mucc VINCENT &FIRST LAWRENCE n POTTS EDWARD E YENDALL I Qom ATTORNEY M 4% M 5:3 H A rcz m 5 S 52.6 sv 2.380

A atent 3,959,439 Patented Oct. 23, 1962 htice 3,059,439 PROCESS ANDAPPARATUS FOR SEPARATING GAS MIXTURES Vincent E. First, Tonawanda,Lawrence D. Potts, Eggertsville, Philip K. Rice, White Plains, andEdward F. Yendall, Kenmore, N.Y., assignors to Union CarbideCorporation, a corporation of New York Original application Jan. 4,1956, Ser. No. 557,260, new Patent No. 2,908,144, dated Oct. 13, 1959.Divided and this application Aug. 24, 1959', Ser. No. 835,536

9 Claims. (Cl. 62-13) This invention relates to an improved process andapparatus for the low-temperatures separation of low boiling point gasmixtures such as air, for example, into higher and lower boiling pointcomponents, and it particularly concerns improvements resulting in thecontinuous removal from separation systems of higher boiling impuritiesseparated in such systems by refrigeration. This invention is a divisionof Serial No. 557,260, now U.S.P. 2,908,144, filed January 4, 1956-.

Gas mixtures containing higher boiling point impurities, such as air,which contain water vapor and carbon dioxide, must be freed of suchimpurities when the gas mixture is to be separated into components bylow temperature rectification. An economical method of eliminating thesehigher boiling impurities while efficiently cooling the gas mixture isto employ periodically reversed heat exchange units having a path inwhich gas mixture being cooled is passed in one direction therethroughand in which separation product being warmed passes in the oppositedirection therethrough when the gas mixture has ceased the flow therein,the gas mixture flow and separation product flow being periodicallyinterchanged to effect alternate deposition along the fiow path of amajor part of the so-called higher boiling impurities from the gasmixture and re-evaporation of such deposited impurities into theoutfiowing separation product. This is of particular economy when thegas mixture does not need to be compressed to high pressure causingexcessive compression energy loss from flow reversal blowdown. However,one of the difficulties that arises in the use of such periodicallyreversed heat exchange units is that when the infiowing mixture andoutflowing product are in balance, such higher boiling point impuritiescannot normally be removed at the rate at which they are deposited inthe heat exchange path and hence gradually accumulate in and block suchpath.

It is known that in such systems the temperature difference betweenincoming air and outgoing separation product increases toward the coldend of the heat exchange unit because of the greater specific heat ofthe compressed air at the lower temperatures. As a result of thistemperature difference, there is also such a dilference in the vaporpressure of the impurities during condensation and re-evaporation thatthe condensate cannot be removed by outgoing product completely at therate at which it was deposited. This undesirable condition can beremedied by passing a greater mass of cold outflowing gas than incomingair through at least the colder part of the heat exchange unit to offsetthe greater specific heat of the compressed air and to reduce thetemperature difference between the air and outfiowing gas in the regionof impurity deposition. In this way the temperature of the separationproduct passing through the reversing heat exchange path will besufiiciently close to the temperature of the air passed previouslytherethrough that it has sufficient capacity to reevaporate all thecondensate deposited therein.

The periodically reversing heat exchange unit is generally of twovarieties: a reversing passage heat exchanger and a cold accumulator orregenerator. As herein used,

a heat exchange unit is intended to comprehend a single reversingpassage heat exchanger or regenerator of tWo or more or such deviceswhich together form a flow arrangement for effecting heat exchangebetween incoming air and outgoing products.

The general purpose of this invention, therefore, is to provide animproved method of thermally unbalancing air cleaning and coolingperiodically reversed heat exchange units of low temperature airseparation systems by utilization of refrigeration available in a bodyof stored cold liquid accessible thereto.

A specific object of this invention is the provision of an improvedmethod of unbalancing such heat exchange units which results insubstantially complete elimination of precipitated higher boilingimpurities from such units continuously during operation of the system.

In accordance with the present invention, unbalance is achieved byutilizing the refrigeration available in a quantity of a cold substanceother than a product of the immediate separation in the cold end of theheat exchange unit to adjust the temperature difference between incomingair and outgoing product. To achieve self-cleaning, the substance addedmust be sufiicient to offset the difference in the specific heats of theincoming air and outgoing product in the region of the heat exchangeunit where carbon dioxide occurs and so effect the reduction in the coldend temperature diiierence required for selfcleaning; i.e. completere-evaporation in the reversing path of the heat exchanger by outgoingproduct of carbon dioxide precipitated from the incoming air during itspassage through said path.

In one application of the present invention, the mass of outwardlyflowing gas passing through at least the cold end of the air cooling andcleaning heat exchange unit of an air separation system is made toexceed the mass of incoming air passing through the same unit by addingto one of the efiluent streams a quantity of cold gas other than aproduct of the immediate separation but having a composition similar tothe separation product of such stream.

In achieving self-cleaning the outgoing product mass can be made toexceed incoming air mass either continuously or cyclically so long asthe average unbalance is sufficient for self-cleaning. In an oxygenproducing plant, the cold substance is preferably oxygen and is made upwholly or in part of oxygen that is derived from a store of liquidoxygen shipped to the plant from a large central plant and generallymade available for supplying part of the oxygen demand when the plantcannot meet the demand and/or, if the plant has inadequate or nomechanical refrigeration-producing devices, for respectively supplyingpart or all of the low-temperature refrigeration required of the plant.

In one aspect of the invention where stored oxygen liquid is used tosupply refrigeration to the plant, the oxygen can be mixed with theoxygen liquid product of the rectification, reboiled therewith, therebygiving up its refrigeration to the system, and passed with the oxygenmake of the separation through an oxygen product pass in the heatexchange unit in such a manner to increase the outflowing gas mass. Thisresults in greater outgoing gas mass flow than incoming air mass flowthrough the colder end of the heat exchange unit. The amount of liquidoxygen supplied for refrigeration purposes will not in many cases beenough to satisfy the requirements for self-cleaning, and in such cases,additional oxygen must be provided over and above the refrigerationrequirements. Additional oxygen for self-cleaning may be added as a gasor as a liquid, provided in the latter instance that heat is madeavailable to evaporate the liquid. In a preferred practice of theinvention, where liquid addition for refrigeration purposes isinsufiicient for selfcleaning, the balance of excess outgoing gasrequired for self-cleaning is made up in a major portion as oxygen makereturned from thewarm end of the heat exchange system to the reboilingzone of the rectification after passage through the oxygen supply linecompressor and in a minor portion as added liquid.

Alternatively, oxygen may be returned from the warm end of the heatexchange system or from an intermediate point thereof by a separateblower or compressor.

In another aspect of the present invention, the unbalance coldoutflowing gas is made up wholly or in part by evaporation from a coldliquid storage tank. The stored liquid means can be any liquid havingthe necessary refrigeration, such as nitrogen, air, or oxygen, but morecommonly will be liquid oxygen held in a storage tank used inconjunction with the gas separation plant in providing, the gas demandrequired. Vapor available from pump priming can also be used and may beconsideredas part of the same source; viz. evaporation from the storagetank. The storage tank may be used for supplying a product gas duringpeak periods when the separation plant supplies only a base or steadysupply, for supplying liquid to meet at least a part of the externalrefrigeration requirements of the separation system, or both. The vaporavailable from the storage tank may be the normal evaporation resultingfrom heat leak or it may be the normal evaporation together with vaporresulting from the forced evaporation, such as, for example, the vaporresulting from the introduction of a gas of the same composition suchas, for example, warm compressed oxygen in the oxygen consumer supplyline, into the liquid of the storage tank.

Other objects, features and advantages of the present invent-ion willbecome apparent in the following description of the accompanyingdrawings in which: 1

The drawing is a schematic flow diagram illustratin an exemplary airseparation plant adapted for the practice of the present invention.

Referring now to the drawing, the apparatus includes a compressor whichpreferably compresses the air to a pressure of about 70 p.s.i.g. anddelivers the air into a conduit 12 lea-ding to the warm end of a pair ofswitching regenerators 100 and 102 for cooling incoming air withoutgoing products. Compressed air in supply line 12 is passedalternately by reversing valves RV through regenerators 100 and 102,with the off-stream regenerator being purged and cooled :by outflowingnitrogen product. Out-going oxygen is preferably divided and passed incontinuous flow through both regenerators in embedded pas-' sages 106and 108. Air which has been freed of moisture and carbon dioxide in there enerators is conducted by conduit 26 through an adsorbent trap 24wherein residual hydrocarbon impurities and carbon dioxide particles are{removed and thence to the lower end of the high pressure rectificationcolumn 28 of a two-stage rectification column indicated generally at 30.V

The air rectification apparatus may be of customary construction and, inaddition to the high pressure rectification or lower column 28, includesan upper, low pressure rectification column 32 having a rectifyingsection 34 and a reboiler section 36 at the lower end thereof whichcloses the high pressure column 28 and separates it from the uppercolumn. The rcboiler 36 operates in a customary manner to condensevapors rising in column 28, producing reflux for the high pressurecolumn and also liquid nitrogen that is collected on 'a shelf 38directly under part of the reboiler 36. The liquid nitrogen from theshelf 38 is throttled to a lower pressure and passed to the top of theupper column through a conduit 40 having a throttle valve 42.Oxygen-enriched liquid collected in a kettle 37 of the lower column istransferred to an intermediate part of the upper column 32 through aconduit 44, the higher pressure liquid being \hrottled 'to a lowpressure by throttle valve 46 in conduit 44 before it is delivered intothe upper column. The liquids transferred to the upper column providereflux and are further rectified to obtain a'cold, gaseous nitrogenproduct at the top part thereof and an oxygen product of desired purityat the lower part thereof. Oxygen-enriched liquid falling from thelowermost tray of the upper column passes into a liquid oxygencollecting space 48 of the reboiler 36 wherein it is vaporized by heatexchange with condensing fluid on the tube side of the reboiler. Part ofthis vapor is withdrawn from the rectification apparatus as oxygen-makeproduct through conduit 50 which delivers it to the cold end of theregenerators and 102 for passage through passageways 1G6 and 108 andthen into the oxygen make supply line 110 having a compressor 112interposed therein for raising the pressure of the gaseous oxygen toconsumer line pressure. The remainder is passed into the rectifyingsection 34 to provide refluxing vapor for the low pressurerectification. The nitrogen product of the rectification is withdrawnfrom the upper column 32 through a conduit 52, which delivers the cold,gaseous efiiuent to check valves 15 at the cold end of the regenerators100 and 102. The nitrogen product may be passed through conduit 52 andheat exchanger '54 to superheat the nitrogen eifiuent and at the sametime subcool the shelf nitrogen liquid sufiiciently to flash-off when itis throttled to a lower pressure in valve 42. The rectification chambers28 and 34 may be provided with the usual gas and liquid contactsurfaces, such as trays 58.

Although it has been described that the outgoing oxygen stream is splitand passed through both regenerators simultaneously it should beunderstood that all of the outgoing oxygen could be passed into a singlestream alternately through one regenerator and then the other, either inphase with the switching cycle of the nitrogen and air or out of phasetherewith. For example, the outgoing oxygen stream can be passed througheach regenerator during the last half of the air-in-fiow half cycle andduring the first half of the nitrogen-out-fiow half cycle. For thesepurposes there is provided control valves 106a and 10811, the openingand closing of which may be timed to effect any desired oxygen flow.

Part of the warm compressed oxygen product is returned as requiredeither to the oxygen reboiler 36 or to the storage tank to providesufficient cold oxygen gas to satisfy the heat exchange unbalancingneeds of the plant. For this purpose there is provided a conduitcontrolled by valve 171 through which a part of the warm oxygen productis'withdrawn from the oxygen supply conduit 110 after passage throughthe oxygen make compressor 112. The Withdrawn gas is passed in heatexchange with the oxygen make stream prior to compression in a heatexchanger 173 and is then partly recooled by passage through coils 174and 176 in the warm end of the regenerators 100 and 102, respectively,the warm oxygen passing through such warm ends in the same direction ofhow as that of the air flow. The flow through the regenerators may be inparallel as shown or it may be alternately through one regenerator andthen the other. The partly recooled clean oxygen gas is then deliveredeither into the oxygen reboiler'36 by conduit 173a having control valve173b or into the storage tank 160 by conduit 173 which terminates belowthe liquid level of the tank so as to cause the relatively warm gas tobubble through the liquid in the tank and thereby increase its rate ofevaporation. When flow of return oxygen is to the reboiler, valve 173ain conduit is closed. Storage tank evaporation, together with oxygenmake gas circulated through the storage tank, is added to the productsof the separation by passage directly into the oxygen make conduit 50 orby delivery into the gas phase of the oxygen reboiler 36 as shown. Inthis way a positive supply of unbalance gas in excess of the oxygen usedfor supplying low temperature refrigeration is made available formeeting the unbalance needs of the separation system. By bubbling thewarm compressed oxygen through the cold liquid oxygen in the storagetank, the amount of refrigeration lost in achieving unbalance isminimized, for the excess cold gas needed is supplied in a minor portionas evaporated liquid and in a major portion of warm compressed gas whichis recooled to saturation by the liquid evaporation.

In the embodiment of the drawing, liquid from the storage tank 164) isfed to the rectification reboiler 36 for refrigeration purposes by apump 1'76 and liquid oxygen from an external source is introducedthrough conduit 180 having control valve 181 therein. It should beunderstood that the low temperature refrigeration supplied by thefeeding of stored liquid to the rectification is not necessarily theentire low temperature refrigeration requirements of the plant but mayform only a part of such requirements as in the case where mechanicalrefrigeration producing apparatus is also present in the system tosupply part of the refrigeration requirements.

It should also be understood that the stored liquid which is supplied tomeet part of or all the low-temperature refrigeration required by therectification need not be stored in the same body or tank as thatsupplying the excess gas to meet peak demands. Hence, as used herein,storage body is intended to comprehend both storage bodies if separateones are used. The term storage body is also intended to cover anoversized reboiler, for cold liquid from a separate source could beintroduced directly into and stored in such a reboiler. Changes in thedetails of the systems herein disclosed may be made without departingfrom the scope of the invention as defined in the following claims.

What is claimed is:

1. In a process for the low temperature separation of air in arectification zone in which air to be cooled and nitrogen product to bewarmed are alternately passed in opposite directions through a reversingfiow path in a heat exchange zone to effect alternate deposition alongthe flow path of water and carbon dioxide impurities from the inflowingair and re-evaporation of such deposited impurities into the outfiowingnitrogen product and in which oxygen product to be warmed is passedconcurrently with the nitrogen product flow through a nonreversing flowpath in said heat exchange zone, the improvement comprising the steps ofproviding an independent storage body of liquid oxygen received from asource separate from the rectification; utilizing a first oxygen liquidfrom said body to supply low temperature refrigeration to therectification zone thereby vaporizing at least a portion of the firstoxygen; withdrawing a second oxygen liquid from said independent storagebody and passing such liquid to a consumer supply line when the oxygendemand exceeds the oxygen separated in the rectification zone;withdrawing oxygen vapor from the rectification zone, said oxygen vaporbeing supplied at least in part by vaporized first oxygen liquid;passing said oxygen vapor through said non-reversing flow path along atleast the colder region of said heat exchange zone as said oxygenproduct which is thereby warmed; withdrawing a portion of the warmedoxygen vapor and recooling such vapor portion by bubbling through theliquid in said independent storage body.

2. A process according to claim 1 in which the withdrawn portion ofoxygen vapor is passed through a separate path in the heat exchange zoneextending along at least the warmer region thereof prior to saidrecooling.

3. A process according to claim 1 in which oxygen vapor resulting fromevaporation of liquid in said independent storage body is passed throughat least the colder region of said heat exchange zone in saidnon-reversing flow path.

4. A process according to claim 1 in which said second oxygen liquid ispumped to consumer line pressure and vaporized prior to said passing tosaid said consumer line, and oxygen vapor resulting from evaporation of6 liquid in said independent storage body is passed through at least thecolder region of said heat exchange zone in said non-reversing flowpath.

5. Apparatus for the low temperature separation of air by rectificationcomprising a rectification column; a reversible heat exchange zonehaving at least two reversing passageways through which air to be cooledand nitrogen product to be warmed are alternately flowed in oppositedirections to effect deposition along the passageways of water andcarbon dioxide impurities from the incoming air and re-evaporation ofsuch impurities into the outfiowing nitrogen product, and at least onenonreversing passageway thermally associated with at least one of saidreversing passageways for Wanming a first oxygen product by heatexchange with the nitrogen product; an independent storage body ofliquid oxygen received from a source separate from the rectificationcolumn; means for withdrawing a first oxygen liquid from saidindependent storage body and passing such liquid to said rectificationcolumn to supply low temperature rerigeration therefor andsimultaneously vaporize at least a portion of said first oxygen; meansfor withdrawing oxygen vapor from said rectification column and passingsuch vapor to the cold end of said non-reversing passageway as saidfirst oxygen product; means for discharging and dividing the warmedfirst oxygen product from said non-reversing passageway; and means forintroducing a divided portion to said warmed first oxygen product in thelower part of said independent storage body of liquid oxygen forbubbling therethrough.

6. Process according to claim 1 including the steps of compressing saidoxygen product after said passage through the non-reversing path andprior to said withdrawal and said bubbling, and partially recooling thecompressed withdrawn oxygen product portion by heat exchange with theuncompressed oxygen product.

7. Process according to claim l'including the steps of compressing saidoxygen product after said passage through the non-reversing path andprior to said withdrawal and said bubbling; directing a first part ofthe compressed oxygen product to said consumer supply lines; partiallyrecooling a second part of the compressed oxygen product as thewithdrawn portion by heat exchange with the uncompressed oxygen product.

8. Process according to claim 7 wherein the second part of saidcompressed oxygen product is further recooled by heat exchange with theWarm-end of said non reversing flow path, and a portion of the recooledsecond part is passed to :the rectification.

9. Apparatus according to claim 5 including means for compressing thewarmed first oxygen product after fiow through said non-reversingpassageway; means for at least partially cooling the compressed firstoxygen product by heat exchange with the uncompressed warm first oxygenproduct; means for further cooling at least a part of said compressedfirst oxygen product in said reversible heat exchange zone; and meansfor passing at least a portion of the compressed and further cooledfirst oxygen product to said rectification column.

References Cited in the file of this patent UNITED STATES PATENTS2,513,306 Garbo July 4, 1950 2,664,718 Rice Ian. 5, 1954 2,690,655Etienne Oct. 5, 1954 2,741,094 Schuftan Apr. 10, 1956 2,785,548 BeckerMar. 19, 1957 2,881,595 Fetter-man Apr. 14, 1959 FOREIGN PATENTS 884,203Germany June 11, 1954 968,603 France Dec. 1, 1950

